1. Field of the Invention
The present invention relates to a differential current driver and a method of using the differential current driver to transmit data.
2. Description of the Related Art
It is known art to use low voltage differential signaling to transmit data between communication devices or large-scale integrated circuit (LSI) devices. This type of transmitting and receiving of data will be described with reference to FIG. 12. Data transmission is carried out by a differential driver 1 that switches output current between two output terminals (referred to as positive and negative terminals) of the transmitting device 40. The current is conducted by a twisted pair cable 45 to the receiving device 50, where it generates voltages at corresponding (positive and negative) input terminals linked by a terminating resistor (not shown). A receiver 51 in the receiving device 50 compares the voltages at these input terminals in order to determine the value of the received data (Rcv_DATA).
FIG. 13 shows the internal structure of the differential driver 1. The differential driver 1 includes a p-channel metal-oxide-semiconductor (PMOS) transistor 2 that operates as a current source (and will be referred to below as a current source 2), PMOS transistors 3 and 4 (referred to below as switches 3 and 4), NAND gates 8 and 9, and inverters 6 and 7. The switches 3 and 4 switch the current output from the current source 2 between the positive (POS) and negative (NEG) output terminals according to a binary data signal received at a data input terminal. The inverters 6 and 7 invert the data signal. The NAND gates 8 and 9 control switches 3 and 4 according the data signal, the inverted data signal, and an output enable (OE) signal received at an output enable input terminal for output enable/disable control of the differential driver 1. When the OE signal is high, one of the two switches 3 and 4 is turned on according to the data signal, and current is output from the corresponding output terminal. When the OE signal goes low, both switches 3 and 4 are turned off, and no current is output.
When the differential driver 1 is switched from the output disabled state to the output enabled state, it takes time for the operation of the current source 2 to stabilize. For that reason, in a device that transmits and receives data at high speed, the internal structure of the differential driver 1 may be altered as shown in FIG. 14. In FIG. 14, when the differential driver 1 is in the output disabled state, a PMOS transistor 14 (referred to below as a switch 14) is turned on, allowing the current from the current source 2 to escape to ground. Current therefore flows from the current source 2 at all times, irrespective of the output state of the differential driver 1. Differing from the structure in FIG. 13, the structure in FIG. 14 eliminates the need to wait for stabilization of the current source 2 when the differential driver is switched from the output disabled state to the output enabled state, and high-speed data transfer becomes possible. These structures are disclosed in Japanese Unexamined Patent Application Publication Nos. 8-204557 and 2000-332610.
In the structure shown in FIG. 13, when the differential driver 1 is in the output disabled state, since the switches 3 and 4 are both turned off, the voltage at their common node N is substantially equal to the power supply voltage (VDD). When the differential driver 1 transitions from this state to the output enabled state, since one of the switches 3 and 4 is turned on, the voltage at the common node N decreases. This node is the also drain node of the current source 2, however. Since the gate and drain of a PMOS transistor are capacitively coupled, when the voltage at the common node N decreases (i.e., the drain voltage of the current source 2 decreases), the bias voltage at the gate of the current source 2 also decreases. As a result, the current flow from the current source 2 increases. For this reason, until the bias voltage returns to its normal level, more current than is required flows from the positive output terminal or the negative output terminal.
In the structure shown in FIG. 14, the increase in output current accompanying a transition of the differential driver from the output disabled state to the output enabled state is prevented, but the differential driver consumes much current, since current flows from the current source 2 at all times.